The present invention relates to cellular radio-telephone systems and more particularly to reduction of co-channel interference in fixed and other cellular radio-telephone systems.
In designing a cellular system layout, a land area is divided into cells each of which corresponds to an area most likely to be serviced by a base station transmitter within that area. A frequency plan assigns the system-allocated frequencies to the various cells in the system layout.
In the frequency plan, frequency reuse is normally employed to achieve a system call capacity significantly greater than the total number of allocated channel frequencies. In applying frequency reuse, a carrier frequency is commonly assigned to different system cells which are separated sufficiently from each other to achieve relatively low interference between radio channels in different cells having the same frequency, i.e., low co-channel interference. Directional base station antennas are normally used in the system cells to achieve further reduction in co-channel interference.
In mobile cellular systems, subscriber unit antennas are normally omni-directional because of the mobility of the subscriber units. Generally, co-channel interference has been found to be minimized in mobile cellular systems when the directional antennas of cells having common frequency assignments are oriented in the same direction.
In a fixed cellular system, or in a cellular system having "fixed" and mobile subscriber units, the "fixed" subscriber units are fixed in location and may be shared by multiple subscribers. For example, all of the occupants of a building might share the use of a subscriber unit disposed in a fixed location in the building. Fixed cellular systems are especially applicable to developing countries and rural areas in developed countries where inadequate wireline capacity exists to provide common wire telephone service. Thus, fixed cellular systems offer an opportunity to achieve quality telephone service quickly without the high capital costs of wire installation. Cellular systems also may have a mix of fixed and mobile subscriber units.
It is desirable that fixed and other cellular systems be provided with frequency plans that are more effective than the conventional frequency plans of the prior art.